(1) Field of the Invention
The present invention relates to a control of an air-fuel ratio in an internal combustion engine. More particularly, the present invention relates to a control of an air-fuel ratio for reducing nitrogen oxide (hereinafter referred to as "NO.sub.x ") in an exhaust gas.
(2) Description of the Related Art
As the conventional apparatus for controlling an air-fuel ratio in an internal combustion engine, there can be mentioned, for example, an apparatus disclosed in Japanese Patent Application Laid-Open Specification No. 203828/84.
According to this technique, the intake air flow quantity Q and engine rotation number N are detected, the basic fuel injection quantity is set based on the detected intake air flow quantity and engine rotation number, and the basic fuel injection quantity is corrected based on the temperature of engine-cooling water and the like factors.
Furthermore, an air-fuel ratio sensor for detecting the air-fuel ratio of an air-fuel mixture supplied to the engine by detecting the oxygen concentration in the exhaust gas is disposed, and under predetermined driving conditions the fuel injection quantity is feedback-controlled based on the detected oxygen concentration so that the air-fuel ratio becomes an aimed value (for example, the theoretical air-fuel ratio). At the time of starting or under high-load conditions, the above-mentioned feedback control is stopped and a feed forward control is performed so that the air-fuel ratio is corrected to a richer value.
Incidentally, in the above-mentioned conventional air-fuel ratio sensor, for example O.sub.2 sensor, an oxidation catalyst layer therein has no substantial effect of reducing nitrogen oxides NO.sub.x, and therefore, the oxygen concentration in the exhaust gas is detected irrespectively of the concentration of nitrogen oxides NO.sub.2. Nitrogen oxides NO.sub.x, however, are formed by bonding of nitrogen N.sub.2 in the air to oxygen O.sub.2 in a high temperature atmosphere.
Namely, O.sub.2 should be detected as O.sub.2, which has not made any contribution to combustion, for detection of the air-fuel ratio, but this oxygen O.sub.2 is not detected by the conventional O.sub.2 sensor.
Accordingly, the detection value of the O.sub.2 sensor is increased by the amount corresponding to the amount of oxygen which is reacted with nitrogen gas N.sub.2 to form NO.sub.x, and in the air-fuel ratio region where the detection value of the O.sub.2 sensor is inverted, the apparent air-fuel ratio is leaner than the actual air-fuel ratio.
Therefore, if feedback control of the air-fuel ratio is performed according to the detection result based on the air-fuel ratio as a reference in the inversion region of the O.sub.2 sensor, the air-fuel ratio is erroneously controlled to a level leaner than the theoretical air-fuel ratio as the target air-fuel ratio, and there is a risk that oxidation reaction of nitrogen gas is advanced and nitrogen oxides NO.sub.x in the exhaust gas are excessive.
Under driving conditions where the NO.sub.x concentration in the exhaust gas is larger, the above-mentioned air fuel control, should employ so-called exhaust gas recycle (EGR) control for recycling a part of the exhaust gas of the engine into a sucked air of the engine to lower the combustion temperature and hence the NO.sub.x concentration. The EGR control system is well-known in the field of automobile engine technique.
In this conventional EGR control system, the structure is complicated because an EGR passage, EGR control valves and other members disposed in the EGR passage are necessary, with the result that the cost is increased. Moreover, the combustion efficiency is reduced by introduction of the exhaust gas into the fresh air to be sucked in the engine and therefore, the fuel consumption is drastically increased.
Accordingly, it is appreciated that the traditional O.sub.2 sensor is used only in the condition of small amount of nitrogen oxides NO.sub.2 in the exhaust gas since the engine is driven by using the leaner air-fuel mixture to get small fuel consumption.
While another improved O.sub.2 sensor is disclosed in the U.S. patent application No. 117,507 by use in which reaction of nitrogen oxides NO.sub.x is further promoted to eliminate the above-mentioned disadvantages of the conventional O.sub.2 sensor structure and the concentration of oxygen, exclusive of oxygen gas which has not participated in combustion, for example, oxygen gas in CO.sub.2, in a sample gas can be detected more accurately. Therefore it is desirable to use the improved O.sub.2 sensor to reduce the amount of nitrogen oxide NO.sub.x in the exhaust gas of the engine when the large amount of nitrogen oxides NO.sub.x is detected.